1. Field of the Invention
The instant invention relates generally to systems, apparatus, and methods for automatic detection and location of acoustic sources in the presence of high levels of background noise. In particular, the invention relates to a process of detecting and rapidly locating the Personal Alert Safety System (“PASS”) carried by firefighters and other first responders when the PASS is in Alarm Mode.
2. Description of the Related Art
Firefighters and other first responders throughout the US and in many parts of the world carry a Personal Alert Safety System (PASS), a device that produces a loud alarm tone if the user is in peril. The alarm tone is intended to perform two primary functions: (1) notify others that the user is in need of immediate assistance, and (2) assist the rescue operation by providing an acoustic signal that can be located by the rescue team. The PASS device automatically switches from Sensing Mode to Alarm Mode if the user is motionless for thirty seconds. Alternatively, the user can manually trigger Alarm Mode by pressing a push-button.
PASS devices are certified to standards generated by the National Fire Protection Association. For the 2007 edition of NFPA 1982 Standard on Personal Alert Safety Systems (PASS), design requirements for the PASS alarm signal include:                1. PASS shall sound the alarm signal when switched to the Alarm Mode.        2. While in the Sensing Mode, PASS shall sound the alarm signal when activated by the motion sensing component when motion is not detected for (30) seconds, +5/−0 seconds.        3. When activated by the motion sensor, the alarm signal shall be preceded by a pre-alarm signal, which shall sound 10 seconds, +3/−0 seconds before the sounding of the alarm signal.        4. During the alarm signal sounding, all other audible PASS signals shall be rendered inactive.        5. The alarm signal shall have a duration of at least 1 hour at a sound pressure level (SPL) of not less than 95 dBA at a distance of 3 m (9.9 ft).        6. The alarm signal, once activated, shall not be deactivated by the motion detector.        7. Any action to silence the alarm signal and the actual silencing of the alarm signal shall not permit the PASS to remain in the Off Mode.        8. The silencing of the alarm signal shall automatically reset the PASS to the Sensing Mode.        
The NFPA Electronic Safety Equipment Technical Committee is responsible for the NFPA 1982 document, which is reviewed and updated approximately every five years. PASS devices certified to the 2007 Edition of NFPA 1982 generally have different alarm tones, depending on the particular manufacturer of the PASS device. The 2013 Edition of NFPA 1982 will specify and standardize the alarm tone so that all PASS devices will sound the same.
Detection Environment
In addition to the usual visibility, contamination, moisture, and temperature issues surrounding a fire scene, the detection of an acoustic signal must deal with the presence of multiple echoes from the structure; in wave propagation terminology, this is known as a high multipath environment. The problem is particularly difficult in smaller structures with highly reflecting surfaces, such as stairwells with concrete walls or shower stalls with tile walls.
The Pathfinder System developed by Summit Safety solves the multipath problem by use of a continuous-wave (CW) ultrasonic transmitter (Beacon) and a directional receiver (Tracker), which detects waves propagating only from a narrow angle. The system is more fully disclosed in U.S. Pat. No. 6,504,794, entitled “Tracking, safety and navigation system for firefighters” and which issued Jan. 7, 2003, and U.S. Pat. No. 6,826,117, with the same title and which issued Nov. 30, 2004. The user must manually scan the area with the Tracker to determine the direction of the strongest signal, which implies the direction of the shortest path to the Beacon. In order to achieve a narrow receiving beam angle, a receiving sensor must have a minimum width of 5-10 wavelengths. For the Pathfinder Tracker, this requirement necessitates the use of ultrasound to ensure portability. The same approach could be used to detect a PASS device, but the size of the sensor would be prohibitive. For example, the wavelength at 1 KHz is approximately 1.13 feet and at 4 KHz is approximately 3.4 inches; a five-wavelength requirement would mean the sensor width would be a minimum of 17 inches (at 4 KHz) and maximum of 5.6 feet (at 1 KHz). In addition, since the 2007 edition of NFPA 1982 allows sequential alarm tones, a manual scanning operation would need to be very slow to ensure that the loudest section of the PASS tone was present at all scan angles.
U.S. Pat. No. 7,639,147 B2 by Berezowski et al. entitled “System and Method of Acoustic Detection and Location of Audible Alarm Devices” which issued 29 Dec. 2009 describes a system of audio sensing modules that comprise a pre-installed infrastructure inside a building. Each of the audio sensing modules incorporates a single sensor (microphone) to collect a time-based record for signal processing. The maximum SPL (sound pressure level) and the minimum SPL for the recording form the basis for PASS alarm detection: if the minimum SPL is not less than a predetermined threshold level or if the difference between the maximum and minimum SPL is below a predetermined threshold level, the module is unable to reliably detect a PASS device. If the SPL levels pass these two threshold tests, the module then determines if an alarmed PASS device is present by analyzing the frequency content of the signal; if the frequencies match the frequency characteristics of the expected PASS device, the module then identifies the repetition pattern of the frequencies. Only after passing the two threshold tests, the frequency content test and the frequency repetition test does the module report the detection of an alarmed PASS device. According to the patent, the process of “locating” a PASS device is accomplished by having multiple sensing modules distributed throughout the building; while not stated explicitly, detection by a particular module implies that the PASS device has been “located” (i.e., its location is within detection range of the particular module). Unfortunately the accuracy of the “location” would be crude at best: the distressed firefighter could still be at a considerable distance from the module. Furthermore, any rescuers would need a map of the building with the locations of the pre-installed modules identified. In addition, the modules would require either a wired or a wireless RF telemetry link in order to notify personnel outside the building that an alarmed PASS device had been detected.
Non-acoustic technologies have also been proposed for locating firefighters in distress. For example, radio frequency systems have been developed to locate firefighters. Such systems have limited capabilities inside a building due to difficulties in wave propagation resulting from the metal and dielectric materials used in the building construction.